Towards finding a novel constant between local and bulk strength of friction stir processed aluminum alloys

Abstract

Friction stir processing has gained remarkable success in producing ultrafine-grained structures and surface composites. In this context, the primary objective is to establish a linear relationship between local strength (i.e. hardness) and bulk mechanical strength (i.e. tensile strength) of friction stir processed aluminum alloys using experimental investigations on selected alloy system together with data reported in literature sources. Initially, authors generated a linear relation between hardness and strength of friction stir processed aluminum alloys under different cooling conditions. After friction stir processing, recrystallized fine grains were formed and better refinement was achieved in cooling-assisted friction stir processing. Irrespective of grain refinement, the strength and hardness of friction stir processed samples were found to be lower compared to the base metal due to the precipitation phenomenon during friction stir processing. At the same time, hardness and strength improved in cooling-assisted friction stir processing compared to natural-cooled friction stir processing due to better grain refinement going by the parameters of Hall–Petch equation. For friction stir processed samples, relevant constants were found using Hall–Petch equation. The experimental values of hardness and strength were well fitted with the formulated equations due to the formation of a homogeneous fine-grained structure. Also, two novel linear relations were successfully established between hardness and strength with proportionality constants of 1.9 and 2.7, respectively. On the other hand, it was also concluded that it is not possible to establish a linear relation between hardness and strength of surface composites due to structural inhomogeneity and agglomeration of reinforcement particles.